This invention relates to the construction of bourdon tubes for use as elastic members of a pressure or temperature sensing device. The bourdon tube as invented by E. Bourdon--U.S. Pat. No. 9163--is basically a tube, flattened and bent in a coil form. When this coiled, flattened tube is secured on one end and subjected to an internal pressure change the coil opens and closes in a near direct proportion to the internal pressure change. The translation of the free end of the bourdon tube is used as an input to a mechanical, electrical or electronic device to indicate and/or control pressure and temperature.
Many factors must be considered in bourdon tube design to meet the requirements of the application. Some factors affecting bourdon tubes are pressure-range, spring-rate, corrosion-resistance to internal fluid, corrosion-resistance to the external environment, repeatability, hysterisis, over-pressure and ambient temperature effect. The bourdon tube shape, size and material are selected for the particular application.
Filled-system thermometers and temperature controllers use bourdons to transduce a pressure change of the internal fluid (liquid, gas or vapor) to a mechanical movement used to drive a pointer or control mechanism. Filled-system thermometers are categorized as three (3) types: Class I--liquid Filled, Class II--Vapor actuated and Class III--Gas Actuated.
Class I utilizes the volumetric expansion of a solid liquid fill to produce a deflection of the bourdon tube with change of temperature. Mercury, zylene and silicone fluids are typical working internal fluids. The bulb size varies inversly with the temperature span. Short spans require larger bulbs, long spans smaller bulbs. The liquid in the bulb expands with increase of temperature and is hydraulically transferred to the bourdon tube through a fine-bore capillary. The volume of the capillary and bourdon are minimized so that ambient temperature effects on the liquid within the capillary and bourdon are minimized. To negate this "ambient temperature effect" several kinds of compensation are employed: (1) a bimetal on the bourdon output which nullifies the "ambient error" at one point (usually mid-span), (2) duplicate capillary with opposing bourdon and (3) capillary filled with invar wire to negate capillary liquid expension.
Class II filled systems have fluids that generate predictable vapor pressure at a given temperature. Increasing bulb temperature vaporizes liquid in the bulb; decreasing temperature condenses saturated vapor in the bulb. The capillary and bourdon volume external to the bulb is filled with liquid when the bulb temperature is above ambient temperature and filled with superheated vapor when the bulb is below ambient temperature. When the bulb temperature increases through ambient temperature liquid transfers from the bulb into the capillary and bourdon, keeping the liquid-vapor interface in the bulb. Larger bulb sizes are required for cross ambient ranges and long capillary lengths.
Class III filled systems are gas-filled and react to changes in bulb temperature with a corresponding change in pressure approximately according to the natural gas law pV=RT. In order to minimize the ambient temperature effect the volume of the bulb is generally 20 to 30 times the volume of the capillary and bourdon tube combined. To obtain narrow temperature spans relatively high internal operating pressures are required and "ambient temperature effect" is greater for higher internal pressures. Therefore, ranges of Class III systems are limited by the elastic range of the bourdon tube and the allowable ambient effect.
Enhanced gas thermal systems overcome the need to have large bulbs by including activated carbon in the bulb to adsorb and desorb gas molecules, amplifying pressure changes beyond that of the natural gas laws. Enhanced gas systems have the advantage that narrow temperature spans can be achieved with lower internal operating pressure and hence lower "ambient effect". This also permits the use of smaller bulbs.
On all temperature and pressure sensing systems employing bourdons made of conventional metallic materials, accuracy is also affected when ambient temperature changes alter the tensile modulus of elasticity of the bourdon. On bourdons made of spring brass, for example, the effect on the spring rate is approximately 0.02% per .degree.F. or 1% per 50.degree. F. ambient temperature change. (Mechanical Measurements, Beckwith & Buch, p.159).
In the case of filled-system thermometer elastic elements, because they are required to operate at elevated pressure spans and hence elevated stress levels, indication errors caused by changes in ambient temperature may be on the order of 3 to 4 times greater.